Electrochromic device based on nanocrystalline materials

ABSTRACT

An electrochromic device, comprising a first electrode ( 3 ), a second electrode ( 5 ) and an electrolyte ( 4 ) separating the electrodes ( 3, 5 ), where at least one of said first and second electrodes includes an electrically active structure which have an at least dual state visual appearance depending on the potential difference between the first and the second electrode.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to an electrochromic device and a methodfor change of state of an electrically active structure in anelectrochromic device. The invention relates more specifically to anelectrochromic device comprising at least one nanostructured electrodewhich does not have an redox active species added to the surface.

TECHNICAL BACKGROUND

Available electrochromic devices may be divided into at least fourclasses:

Firstly, there are devices based on ion insertion reactions atessentially dense metal oxide electrodes. To ensure the desired changein transmittance needed to bring about a color change, a certain numberof ions must be intercalated into the electrode to compensate for theaccumulated charge. However, since the surface area in contact with theelectrolyte is not significantly larger than the geometric area of theelectrode, the use of metal oxide electrodes requires bulk intercalationof ions. As a consequence, the switching times of such a device aretypically of the order of tens of seconds. The charge required for thecoloration is counterbalanced by oxidation of a redoxcomponent, such asferrocene or the like, in the electrolyte at the counter electrode, orby oxidation of a NiO_(x)H_(y) film on the counter electrode.

In U.S. Pat. No. 4,561,729 an electrochromic indicating device of thistype is presented, which comprises a counter electrode comprised ofactivated carbon. This counter electrode counterbalances the chargerequired for the coloration of a coloring electrode of intercalationtype by capacitive charging. In U.S. Pat. Nos. 5,940,202 and 5,708,523,similar electrochromic devices are shown, each comprising a porouscarbon counter electrode, which is formed in a linear pattern and adotted pattern, respectively.

Secondly, there are solution-phase electrochromic devices, in which oneor two electrochromic compounds are dissolved in an electrolyte betweentwo electrodes. Such systems are disclosed in U.S. Pat Nos. 4,902,108and 5,128,799, wherein a molecule which colors by reduction and anotherone which colors by oxidation are present in the solution. Theapplication of a voltage over the electrodes then results in thereduction of the former substance at the cathode and oxidation of thelater at the anode. Systems of this type are not bistable and thereforebleach spontaneously when the current is off.

Thirdly, there are devices based on a transparent conducting substratecoated with a polymer to which a redox chromophore is bound. On applyinga sufficiently negative potential there is a transmittance change due toformation of the reduced form of the redox chromophore. To ensure thedesired change in transmittance a sufficiently thick polymer layer isrequired, the latter implying the absence of an intimate contact betweenthe transparent conducting substrate and a significant fraction of theredox chromophores in the polymer film. As a consequence the switchingtimes of such a device are, as for the first type discussed above,typically of the order of tens of seconds.

Fourthly, there are devices wherein at least one of the electrodescomprises a (semi-) conducting nanocrystalline film. The nanocrystallinecoloring electrode is in this case formed of a metal oxide which carriesa monolayer of adsorbed electrochromophoric molecules. These moleculescomprise firstly an attachment group and secondly an electrochromophoricgroup, which do not absorb visible light in the oxidized state, but doesabsorb light in the reduced state (type n electrochromophore). More indetail, the extinction associated with the reduction of one type nelectrochromophore is in the order of a magnitude higher than that ofone electron in the nanostructured metal oxide. Further, due to thenanoporous structure and associated surface roughness of thenanocrystalline films used, the redox chromophore is effectively stackedas in a polymer film, while at the same time maintaining the intimatecontact with the metal oxide substrate necessary to ensure rapidswitching times. Alternatively, the electrochromophoric group may be atype p electrochromophore, whereby it exhibits a reverse behavior, i.e.it does absorb light in the oxidized state, but it does not absorb lightin the reduced state. More generally, the electrochromophoric group mayhave two or more oxidation states with different colours. In such casesit is not possible to classify the electrochromophore as n-type orp-type.

A “nanocrystalline film” is constituted from fused nanometer-scalecrystallites. In a “nanoporous-nanocrystalline” film the morphology ofthe fused nanocrystallites is such that it is porous on thenanometer-scale. The porosity of a nanostructured film is typically inthe range of 50-60%, and the particle size is typically within the rangeof from a few nanometers up to several hundred nanometers in at leasttwo dimensions (i.e the particles may be shaped as spheres, rods,cylinders e.t.c.). The thickness of a nanostructured film is typicallyin the order of 5-10 μm, but may be up to several hundred μm. Suchfilms, which may hereinafter be referred to as nanostructured films,typically possess a surface roughness of about 1000 assuming a thicknessof about 10 μm. Surface roughness is defined as the true internalsurface area divided by the projected area.

The basic concept of nanostructured thin films is described by B.O'Regan and M. Grätzel in Nature, 353, 737 (1991), and by Grätzel et alin J. Am. Chem. Soc., 115, 6382 (1993).

The application of nanostructured thin films in electrochromic devicesis described by A. Hagfeldt, L. Walder and M. Grätzel in Proc. Soc.Photo-Opt. Intrum. Engn., 2531, 60 (1995), and by P. Bonhôte, E.Gogniat, F. Campus, L. Walder and M. Grätzel in Displays 20 (1999)137-144.

In their article Bonhôte et al disclose an electrochromic system whereina nanostructured electrode with a type n electrochromophore added to thesurface is used in conjunction with a metal counter electrode (e.g. Zn)and ions of the same metal in the electrolyte. The charge needed forcoloration of the nanostructured electrode is achieved by oxidation ofthe metal resulting in dissolution of metal ions and subsequentbleaching by redeposition of the metal. Long-term stability appears tobe poor as prolonged cycling leads to metal particle and dendriteformation. It is also known that such electrochromic devices exhibitproblems with bubble formation. This may be solved by a special chemicaltreatment that also adds requirements on the composition of theelectrolyte. The unstable metal surface is further not a good substratefor additional films, e.g. a white reflector. Metal ions may also bedeposited in the coloring electrode or in other locations in thedisplay, causing unwanted irreversible blackening.

In WO9735227 (U.S. Pat. No. 6,067,184) P. Bonhôte et al lay forwardseveral electrode-combinations for electrochromic devices comprising oneor two nanostructured electrodes with electrochromophores added to thesurface. In a first embodiment a nanostructured electrode with a type nelectrochromophore added to the surface, is used in conjunction with atransparent material of polymeric type, which is oxidizable, colorlessin the reduced state and, respectively, colorless or colored in theoxidized state. In another embodiment, both electrodes arenanostructured and have adsorbed molecules on the surface, type nelectrochromophoric on the cathode and type p electrochromophoric on theanode, whereby a device with two coloring electrodes is achieved. Athird proposed embodiment comprises a nanostructured coloring-electrode(anode) with adsorbed molecules on the surface (type pelectrochromophoric), and a nanostructured counter electrode with noadsorbed molecules. In this device, the charge required for thecoloration is counterbalanced by insertion of small cations into thenanostructured counter electrode

In WO9835267 Fitzmaurice et al disclose that a nanostructured film, ifused without the adsorbed redox chromophores, becomes colored onapplication of a potential sufficiently negative to accumulate electronsin the available trap and conduction band states. As a consequence ofthe high surface roughness of these films, ions are readilyadsorbed/intercalated at the oxide surface permitting efficient chargecompensation and rapid switching, i.e. the need for bulk intercalationis eliminated. However, Enright et al mentions that, despite the rapidswitching times in such films, the associated change in transmittance isnot sufficient for a commercial device. To overcome this limitation theypropose that redox chromophores are adsorbed at the surface of thetransparent nanostructured film, just as in the devices described above.This coloring electrode is then used in conjunction with an electrolytecontaining ferrocene and a conducting glass counter electrode. Uponapplication of a voltage, reduction takes place at the coloringelectrode and oxidation of ferrocene at the counter electrode. Asreduced and oxidized species react internally, a permanent applicationof voltage is required to maintain the colored state i.e. there is nomemory-effect. Naked electrodes, that is electrodes which does not havean redox active species added to the surface, have not been consideredin previous displays with electrochromic capacitive electrodes, neitheras non-colouring counter electrodes nor as counter electrodes, sincenaked electrodes has been considered to have too low charge capacity andcolouration efficiency to be used in displays.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an electrochromicdevice which, compared to prior art, exhibits rapid colour change, has agood memory effect, has improved stability, exhibits black colourationand which may be prepared using low cost processes.

These and other objects of the invention are achieved by the inventionas defined in the independent claim 1. By arranging at least one of saidfirst or second electrodes as a nanostructured conducting orsemiconducting film, which does not have an redox active species addedto the surface and furthermore arranging said at least one electrodetogether with the electrolyte for supporting a capacitive chargecompensation between ions in the electrolyte and said at least oneelectrode, the change of state of the electrically active structureoccurs without transfer of charge between the electrolyte and theelectrodes and hence the time span for a colour change is reduced incomparison to electrodes only supporting intercalation processes forobtaining a colour change. Furthermore when using naked electrodes thereare a less demand on designing the molecules for obtaining matchingbetween energy levels of the redox active species and the electrodematerial. Furthermore the risk for deattachment of the redox activespecies at the electrode is eliminated by using naked electrodes.Furthermore the time consuming process of attaching redox active speciesin a bath is omitted.

Two different types of reactions may occur at an electrode in aelectrochemic cell. In the first type of reaction a transfer of chargeoccurs between the electrolyte and the electrode. An example of thistype of reaction is the reaction at a Zinc electrode in a battery. Zincfrom the electrode is transferred to Zinc ions migrating to theelectrolyte or vice versa. A current is flowing through the cell.

The other type of reaction is capacitive charge compensation which isdefined as a charging of the interface between the electrode and theelectrolyte by an accumulation of a quantity of positive and negativecharge (electrons, ions holes, etc.) at either side of said interface.For example, accumulation of negative charges in the electrode iscompensated by accumulation of positive charges in the electrolyte, orvice versa. Thus intercalation processes are excluded. The process doesnot involve any chemical reactions or structural transformation for thematerial involved. Further, there is no transport of charge from theelectrolyte to the electrodes or vice versa. An example of a capacitivecharge compensation is the reaction occurring at a porous carbonelectrode. Electrons from a voltage source migrate in and out from theelectrode material depending on the applied voltage, but are nottransferred to the electrolyte. Instead the charge at the carbonelectrode is balanced by ions in the electrolyte migrating towards orfrom a boundary layer surrounding the surfaces of the electrode. Aftercharging the electrode there is no current flowing through the cell.

The definition of capacitive charge compensation used in thisapplication also includes reactions where a transfer of charge existbetween the electrode material and an eventually existing layer, of forinstance electrochromophores, adhered to the electrode as long as thereis no transfer of charge from the electrolyte to the electrode and/orthe material covering the electrode. Furthermore we make no distinctionbetween electrodes with internal charge transfer reactions fromelectrodes with no such internal charge transfer reactions. An electrodeis capacitive if the electrode is due for interfacial charging meaningthat the charging of metal oxide particles, attached molecules etc iscompensated by opposite charging in the electrolyte.

In a first preferred embodiment of the invention the first electrode,second electrode and electrolyte of the electrochromic device supportscapacitive charge compensation by including a first electrode (3) whichcomprises a nanostructured conducting or semiconducting film, which filmhas an electrochromophore added to the surface and a second electrode(5) which is a nanostructured conducting or semiconducting film, whichdoes not have an redox active species added to the surface. In the firstembodiment the electrically active structure is thus constituted by saidan electrochromophore molecules added to the surface.

In a second preferred embodiment of the invention the first electrode,second electrode and electrolyte of the electrochromic device supportscapacitive charge compensation by including a second electrode (5) whichis a nanostructured conducting or semiconducting film, which does nothave an electrochromophore added to the surface, and which film iselectrically active having a dual state visual appearance depending onthe potential difference between the first and the second electrode. Inthe second embodiment the electrically active structure is constitutedby the film itself.

In a first embodiment of said second preferred embodiment a firstelectrode (3) is included, which first electrode is a nanostructuredconducting or semiconducting film, which does not have an redox activespecies added to the surface.

In a second embodiment of said second preferred embodiment a firstelectrode (3) is included, which first electrode is either anon-capacitive electrode or a capacitive electrode having anelectrochromophore added to the surface.

The redox active species are preferably constituted byelectrochromophore molecules.

In a preferred embodiment the electrolyte is electrochemically inert. Byproviding an electrochemically inert electrolyte a good memory effect isobtained since the electrodes together with the electrolyte are capableof retaining a charged boundary layer surrounding the surfaces of theelectrodes being in contact with the electrolyte after removingexternally applied voltage source.

In order to ensure a short time span for a colour change thenanostructured conducting or semiconducting film at the second electrode(5) are formed with a roughness factor of at least 20.

Embodiments of the invention are defined in the dependent claims.

SHORT DESCRIPTION OF THE FIGURES

FIG. 1 shows a schematic picture of an electrochromic device accordingto the present invention

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

An electrochromic device (FIG. 1), e.g. a window, comprises a firstelectrode 3 and a second electrode 5, out of which at least one becomescolored under reducing or oxidizing conditions. The electrodes 3,5 areseparated by an electrolyte 4. Each of the two electrodes is typicallysupported by a conducting transparent plate 1, 7 or the like, e.g. aglass plate covered with a transparent conductive coating 2, 6, such asdoped tin oxide. Furthermore, the conductive coatings 2, 6 are connectedto an external electric circuit by means of contacts 8.

The electrochemical capacitance of a conducting (or semiconducting)surface in contact with an electrolyte is typically about 10 μFcm⁻² to40 μFcm⁻² (the electrical double layer or Helmholtz capacitance). Byusing a nanostructured conducting film with a roughness factor of about1000 (described more in detail below), it has been found that the totalcapacitance is raised to about 10 mFcm⁻² to 40 mFcm⁻² (the relationshipbetween the roughness factor and the capacitance is proportional). Thisraise in capacitance makes it possible to use such a nanostructured filmas an electrode in an electrochromic device, as it has the ability toprovide the charge needed to color a coloring electrode in anelectrochromic device, (which is in the range of 5 mCcm⁻² to 20 mCcm⁻²).

Furthermore, since no intercalation is involved, such an electrode is“fast” enough to be used in any fast color switching electrochromicdevice

In a nanostructured electrode it is essential that the particles areelectrically connected with each other and the conducting substrate.They can be sintered together (heated), pressed together, chemicallyconnected, connected with some kind of inorganic or organic binderparticles in the film, etc. The porosity in the film must be high.Essential is that the pores in the film also form a 3-d network withnano-dimensions (1-100 nm pore size). This open porous structure makesthe ion transport rapid when immersed in an electrolyte.

To improve the conductivity of electrons and ions in the film, the filmmay contain particles of larger size than nanoparticles. For example,micrometer-size ZnO rods that are grown from the substrate, as disclosedin WO 9800035-9 or graphite. In the same way the porous network maycontain micron-size “pore channels” to speed up the ion transport. Onecould also imagine other additives, like light-scattering particles andthe “binder particles” discussed above. Essential is that the maincontact between the electrode and the electrolyte is located at thesurface of the nanoparticles, and that this interface is easilyaccessible (e.g. not via long narrow pores within a particle) from the3d-networks of both the particles and the pores.

Examples of suitable materials for such nanostructured conducting filmsare semiconducting metal oxides, carbon, metals and other semiconductingmaterials. A suitable semiconducting metal oxide may be an oxide of anysuitable metal, such as, titanium, zirconium, hafnium, chromium,molybdenum, tungsten, vanadium, niobium, tantalum, silver, zinc, tin,strontium, iron, cobalt or nickel or a perovskite thereof.

The present inventors have discovered that certain semiconducting metaloxides (specified below), when prepared as nanostructured films with aroughness factor of at least 20, exhibit color-change characteristicsthat are not dependent upon intercalation of ions into the material. Themain mechanism in these cases is instead capacitive charging (or doublelayer charging) at the surface of the nanostructured material. Thiscapacitive behavior leads to much faster color switching, as there isessentially no intercalation involved.

For a nanostructured coloring electrode NiO (in the crystalline formbunsenite), CoO, WO₃ and MoO₃ are particularly preferred. Out of these,NiO and CoO become colored under oxidizing conditions and the othersunder reducing conditions. For a nanostructured non coloring electrodeTiO₂, In₂O₃, SnO₂, RuO₂ and carbon are particularly preferred.

The electrolyte is preferably in liquid form and preferably comprises atleast one electrochemically inert salt, either as a molten salt ordissolved in a solvent.

Suitable salts are composed of cations such as lithium, sodium,potassium, magnesium, tetraalkylammonium and dialkylimidazolium ions,and anions such as chloride, perchlorate, trifluoromethanesulfonate,bis(trifluoromethysulfonyl)amide, tetrafluoroborate andhexafluorophosphate ions.

Suitable solvents are electrochemically inert such as water,acetonitrile, methoxyacetonitrile, butyronitrile, propionitrile,3-methoxypropionitrile, glutaronitrile, -butyrolactone,propylenecarbonate, ethylenecarbonate, dimethylsulfoxide,dimethylformamide, dimethylacetamide, and N-methyloxazolidinone, ormixtures thereof.

Suitable molten salts are e.g. dialkylimidazoliumtrifluoromethanesulfonate and dialkylimidazoliumbis(trifluoromethysulfonyl)amide.

In one preferred embodiment, the first electrode 3 is a nanostructuredelectrode with a type n electrochromophore added to the surface. Thesecond electrode 5 is a non-coloring electrode, comprising ananostructured film of a conducting or semiconducting material asdefined above. It should be noted that this second electrode 5 in thisdevice does not have an adsorbed monolayer of electrochromophore or thelike on the surface, whereby the production step of adding anelectrochromophore to this electrode is omitted. Systems of this typeutilize the fast color switching characteristics of theelectrochromophore and the capacitive behavior of the nanostructuredelectrode. Such a system exhibits as fast color switching as the priorart systems based on electrochromophores, but has substantially betterlong-term stability (and cyclability). These improvements are due to thefact that no electrochemical reactions, other than (pseudo-) capacitivecharging, are taking place at the counter electrode.

In another embodiment both the first electrode 3 and the secondelectrode 5 lack adsorbed monolayers of electrochromophores or the likeon the surface. In this embodiment the second electrode 5 is ananostructured coloring electrode of the type described above, i.e. ananostructured NiO electrode or the like.

In a third embodiment both the first electrode 3 and the secondelectrode 5 are nanostructured coloring electrodes, i.e. one of theelectrodes becomes colored under reducing conditions, and the otherelectrode becomes colored under oxidizing conditions.

As both electrodes in the last two embodiments lack adsorbed monolayersof electrochromophores or the like on the surface, production of suchsystems will be faster and less complicated. The adsorption ofelectrochromophores at the nanostructured electrode is a time consumingstep in the fabrication of nanostructured electrochromic devices. Theadsorption process may also negatively affect the properties of theelectrode material. Such systems will further exhibit enhanced long-termstability since there are no intercalation or electrodepostion reactionsat the electrodes and problems associated with desorption ofelectrochromophores are avoided.

By avoiding intercalation or electrodepostion reactions and adsorbedelectrochromophores, one reduces the risk for competing destructivereactions (electrochemically and photo-induced). Supercapacitors withpure double-layer capacity are generally believed to have the highestelectrochemical stability, in fact, electrochromic devices with twonanostructurednanoporous electrodes are “colouring supercapacitors”.

Even though the color switch in the two last embodiments is notdependent upon intercalation, there will still exist intercalation to,some degree if small ions such as lithium ions are present in theelectrolyte. One way to minimize the intercalation, which may slow downthe color switch process, is to use an electrolyte that does notcomprise such ions. Therefore, it is preferred to use an electrolytethat only comprises larger ions such as for example tetraalkylammoniumions.

The electrolyte thus supports capacitive charge compensation whencapacitive charge compensation processes are dominant in relation toexisting intercalation processes, in particular under change of colourof the electrode.

SPECIFIC EXAMPLE

An electrochromic display according to the invention may be provided asdescribed in detail below.

Bis-(2-phosphonoethyl)-4,4′-bipyridinium dichloride is adsorbed to thesurface of a 4 m thick nanostructured film of TiO2 on a conducting glassplate (0.5 μm fluorine-doped SnO2 on 2 mm glass). This electrode istransparent, but colours blue upon reduction. A nanostructured carbonfilm (10-50 μm thick), comprising carbon black and graphite particles,is deposited on a second conducting plate. On top of this film a porouswhite light-scattering film is deposited as a reflector. The two platesare assembled face-to-face using a hot-melting plastic at the uncoverededges of the two plates. Electrolyte (0.2 M tetrabutylammoniumtrifluoromethanesulfonate in 3-methoxypropionitrile) is introduced inthe space between the two electrodes. The resulting electrochromicdisplay has a good memory effect and stability (>100,000 cycles withoutsevere degradation).

Above a number of embodiments have been described. However, it isobvious that the design could be varied without deviating from theinventive idea, of providing an improved electrochromic device.

Therefore the present invention should not be regarded as restricted tothe above disclosed embodiments, but can be varied within the scope ofthe appended claims.

1. An electrochromic device, comprising a first electrode, a secondelectrode and an electrolyte separating the electrodes, wherein saidfirst electrode comprises an electrically active structure which has anat least dual state visual appearance depending on the potentialdifference between the first and the second electrode; wherein saidsecond electrode comprises a nanostructured conducting or semiconductingfilm, which does not have a redox active species added to the surface;and wherein said second electrode together with the electrolyte arearranged for supporting a capacitive charge compensation by charging ofan interface between said second electrode and the electrolyte by anaccumulation of a quantity of positive and negative charges at eitherside of said interface.
 2. An electrochromic device according to claim1, wherein said electrolyte is an electrochemically inert electrolyte.3. Electrochromic device according to claim 1 or 2, wherein saidelectrically active structure comprises a nanostructured film which hasan electrochromphore added to the surface.
 4. Electrochromic deviceaccording to claim 3, wherein the film is transparent under reducingconditions and become colored under oxidizing conditions. 5.Electrochromic device according to claim 1, wherein the nanostructuredconducting or semiconducting film at the second electrode has aroughness factor of at least
 20. 6. Electrochromic device accordingclaim 1, wherein the nanostructured conducting or semiconducting film atthe second electrode is a nanostructured film of doped or undopedcrystalline nickel or cobalt oxide or a mixture thereof. 7.Electrochromic device according to claim 1, wherein the nanostructuredconducting or semiconducting film at the second electrode is comprisedof carbon.
 8. Electrochromic device according to claim 1, wherein thenanostructured conducting or semiconducting film at the second electrodeis comprised of a metal.
 9. Electrochromic device according to claim 1,wherein the nanostructured conducting or semiconducting film at thesecond electrode is comprised of a semiconducting metal oxide, selectedfrom the group consisting of: an oxide of titanium, an oxide ofzirconium, an oxide of hafnium, an oxide of chromium, an oxide ofmolybdenum, an oxide of tungsten, an oxide of vanadium, an oxide ofniobium, an oxide of tantalum, an oxide of silver, an oxide of zinc, anoxide of tin, an oxide of strontium, an oxide of iron, an oxide ofcobalt, an oxide of nickel and a perovskite thereof.
 10. Electrochromicdevice according to claim 3, wherein the nanostructured film at thefirst electrode is transparent under oxidizing conditions and becomecolored under reducing conditions.
 11. Electrochromic device accordingto claim 10, wherein the nanostructured conducting or semiconductingfilm at the second electrode is a nanostructured film of doped orundoped crystalline molybdenum or tungsten oxide or a mixture thereof.12. Electrochromic device according to claim 10, wherein said capacitivecharge compensation occurs between ions in the electrolyte and at saidsecond electrode.